The present invention relates to reducing the effects of noise in a video graphics system, and more particularly to methods and apparatus for reducing the effects of noise caused by reading data from a memory in a video graphics system.
In a conventional video graphics system, data is provided by a graphics pipeline to a digital-to-analog converter (DAC), the output of which drives the input of a display monitor. Accordingly, noise at the DAC output creates video noise on the display, and degrades its performance. Thus, it is desirable to reduce noise at the DAC output.
One source of noise is ground bounce caused by the circuit switching and other voltage transients in the video graphics system. Also, these transitions often contain high frequency components that may couple to the DAC output. If more circuits switch simultaneously, the resulting ground bounce is exacerbated. Of particular concern is ground bounce caused by reading data from a graphics memory, since data having widths of 64, 128, or more bits wide may be simultaneously read from memory. As memory outputs change state during a read, capacitances on the output lines are charged or discharged. This results in large, short duration current pulses into and out of the ground supply, thereby causing the ground bounce.
If the ground bounce is random, spread in time, or has a low amplitude, the video noise generated is not necessarily apparent to an observer viewing the display. But if the ground bounce is synchronous, that is, periodic such that it occurs each time a particular pixel on the display is being updated, the resulting change in that particular pixel may become noticeable. Moreover, if many adjacent pixels are affected, such as those forming a horizontal or vertical line, an undesirable artifact may result.
Accordingly, prior art solutions have been developed to reduce ground bounce noise. For example, analog design techniques such as filtering or ground plane separations have been used. Unfortunately, these solutions require the use of costly electrical components that consume board space and often require one or more board revisions or spins.
Thus, what is needed are low cost, easily integrated methods and apparatus for reducing the effects of ground bounce and other electrical switching noise on a video signal.